Jun 6th 2015 |

Collimating a Newtonian

Collimation is critical to obtaining the best performance from your telescope. There is a bit of an art to collimating a Newtonian. Once you have done it a few times, it will start to become second nature. Unless you have a truss-tube telescope which must be assembled each time it is used, collimation should be an infrequent necessity. You should be able to go for weeks or months without collimating a telescope under normal use. If you find collimation to be necessary every couple weeks, you may not be properly locking down the mirrors after collimation, or a component may be loose somewhere in the optical train.

Tools

There are two basic tools necessary for accurate alignment of Newtonian telescope optics.

Collimation eyepiece

Laser collimator

Above:A laser collimator and collimation eyepiece

A collimation eyepiece is a metal tube with crosshairs in it. It fits into the focuser just like an eyepiece. Looking through it you see the crosshairs overlaying the telescope optics, allowing you to determine the alignment of the mirrors. A laser collimator also fits into the focuser like an eyepiece. It projects a red laser beam, placing a dot onto the primary mirror which is reflected onto the secondary mirror then back onto the collimator (when everything is properly aligned).

In addition to these items, you will need the appropriate wrenches or screwdrivers to adjust your telescope’s mirrors. A common set of tools would be a Phillips screwdriver and hex-head wrenches for the secondary mirror, and a flat-head screwdriver for the primary mirror, but each telescope is different.

Outline of Procedure

Before beginning, know your plan of attack. There is a correct procedure to use when collimating a Newtonian. The basic steps are as follows:

Align the secondary mirror using the collimation eyepiece

Align the secondary mirror using the laser collimator

Align the primary mirror using the laser collimator

Double check everything with the collimation eyepiece

One of the common misconceptions about collimation is that most of the adjustment must be done on the primary mirror. In fact, the opposite is true. Most of the collimation adjustment is with the secondary mirror, especially in a truss-tube telescope. The secondary mirror is more likely to be bumped out of alignment, and it has more degrees of freedom than the primary mirror.

Visual Alignment of Secondary Mirror

The first step is aligning the secondary mirror using the collimation eyepiece. As mentioned above, do not underestimate the importance of proper secondary mirror alignment. Begin by inserting the collimation eyepiece into the focuser drawtube and take a look through the eyepiece. An out-of-collimation instrument will appear like the diagram below. A properly collimated telescope is shown as well.

Keep in mind that the secondary mirror has three degrees of freedom: tilt, rotation, and longitudinal adjustment. Tilt is controlled by three screws behind the secondary mirror. These screws push against the flat back of the mirror holder. They also lock the mirror in place when collimation is complete. Rotation and longitudinal adjustment (movement up and down the tube along the optical axis) are controlled by a single larger screw in the center of the mirror holder. This screw actually attaches to the mirror assembly, holding in onto the spider vanes.

Below are diagrams depicting the appearance through the collimation eyepiece of errors in tilt, rotation, and longitudinal adjustment. Keep in mind that these errors will normally occur in combination. The first thing to try is adjusting the tilt of the mirror because it is the easiest error to see. Once this is eliminated, you can check for errors in the other degrees of freedom.

Above:Tilt error (see description below)

Above:Rotation error (see description below)

Above:Longitudinal error (see description below)

Tilt error shows up as a displacement of the reflection of the secondary mirror (the dark circle near the center of the view). Correct this by adjusting the three screws pushing the mirror assembly until the reflection of the secondary is centered. At this point, the primary mirror clips should be visible evenly around the perimeter of the view. (There will be three or four depending on the model. These diagrams show four.)

If the secondary reflection and clips are displaced up or down as shown in the second diagram above, rotational adjustment is necessary. After rotating the mirror, readjust the tilt as well.

Finally, if the focuser drawtube is not centered in the secondary mirror reflection, while everything else appears centered, a longitudinal adjustment is required. In the third diagram above, the secondary mirror sits too far down the tube (too close to the primary) and needs to be moved up. This is done by loosening the tilt screws and tightening the main screw. After this it will be necessary to readjust the tilt once more.

Laser Alignment of Secondary Mirror

The next step is to refine the tilt alignment of the secondary mirror using the laser collimator. Insert the laser into the focuser and turn it on. Look down the front of the tube at the primary mirror. You should see the laser produce a red dot on the primary. This dot should coincide with the dot marking the center of the primary mirror. If there is no center dot on your mirror, use the distance of the laser dot from the mirror clips to estimate the center. Adjust the tilt of the secondary mirror, if necessary, to place the red dot directly on the center of the primary.

Above:Misaligned secondary produces a red dot offset from the primary mirror center. Adjust the mirror until the dot hits the center mark of the primary.

Confirm Alignment

Before proceeding to the primary mirror, be sure to double check the alignment once more using the collimation eyepiece. This will verify that tilting the secondary to align the laser dot did not introduce any rotational error. If needed, repeat the rotation and tilt adjustments until everything is perfect.

Laser Alignment of Primary Mirror

Finally, the primary mirror can be adjusted. You can take a quick look through the collimation eyepiece to see what the appearance of the optics are when the secondary is aligned but the primary is not. The general appearance is that the primary mirror center dot and mirror clips are centered on the crosshairs of the eyepiece, but the reflection of the secondary mirror itself is offset.

Above:Properly aligned secondary with unaligned primary

Insert the laser into the focuser drawtube and turn it on. This time, look for the reflection of the secondary mirror. You should see two red dots. One comes from the bottom of the laser itself, and the other is a reflection of the laser beam back onto the bottom of the collimator (or onto the focuser if the alignment is way off). The trick now is to get the two dots to coincide, sending the laser beam right back on itself.

Above:Laser dots produced by a misaligned primary

Most primary mirrors have six screws on the mirror cell: three push screws and three pull screws. The push screws are normally smaller and lock the mirror in place. The push screws are usually bigger and have springs on them. These control the main adjustment of the mirror.

Above:Collimation screws on a typical Newtonian

Loosen the lock (push) screws to allow the mirror to be adjusted. Unless you have a helper or a very short telescope, you will have to make an adjustment to the mirror then walk up to the front of the scope and see whether things got better or worse. Continue this until the two red dots perfectly coincide. Keep the laser on while you tighten the lock screws. Tighten each screw about a quarter turn at a time and continue working around until all three are snug. This will tend to keep the mirror in alignment while you lock it down. Double check the laser again after tightening the lock screws.

Confirm Alignment

Once again, double check the alignment with the collimation eyepiece to be sure everything looks okay.

Above: Appearance of a perfectly collimated Newtonian through a collimation eyepiece